West Nile virus

noun

variants or less commonly West Nile

1

: a flavivirus (Orthoflavivirus nilense) that causes an illness marked by fever, headache, muscle ache, skin rash, and sometimes encephalitis or meningitis and that is spread especially from birds to humans by mosquitoes

2

or West Nile fever : the illness caused by West Nile virus

Examples of West Nile virus in a Sentence

Recent Examples on the Web

The first Bay Area case of West Nile virus this year was confirmed last month in Santa Clara County. — Jason Green, Mercury News, 21 Aug. 2026

But officials have found West Nile virus in mosquitoes across every borough of New York City this summer—and that’s not unusual. — Mary Randolph, Scientific American, 21 Aug. 2026

West Nile virus disease is caused by bites from infected mosquitoes. — Matthew Rodriguez, CBS News, 21 Aug. 2026

At a state level, around 24% of dead birds identified through surveillance testing are testing positive for West Nile virus this year, compared with the five-year average of 13%, according to the state Department of Public Health. — Clara Harter, Los Angeles Times, 21 Aug. 2026

Word History

Etymology

from West Nile province of Uganda, where the virus was isolated in 1937

First Known Use

1940, in the meaning defined at sense 1

Rhymes for West Nile virus

• adenovirus
• baculovirus
• coronavirus
• enterovirus
• poliovirus
• antivirus
• herpesvirus
• parvovirus
• reovirus
• retrovirus
• rhinovirus
• rotavirus

Medical Definition

West Nile virus

noun

variants also West Nile

1

: a flavivirus (Orthoflavivirus nilense) that causes an illness marked by fever, headache, muscle ache, skin rash, and sometimes encephalitis or meningitis, and that is spread especially from birds to humans by mosquitoes chiefly of the genus Culex

Note: West Nile virus is closely related to the viruses causing Japanese encephalitis and Saint Louis encephalitis.

West Nile virus causes unusually severe infections in crows and blue jays, according to one CDC-led study that measured the number of virus particles in the birds' blood …

—Stephen S. Hall, Smithsonian

Of 5780 cases of arboviral disease reported in 2012, West Nile virus infection accounted of 5674, or 98% of the total, researchers reported in the Morbidity and Mortality Weekly Report.

—Rebecca Voelker, The Journal of the American Medical Association

2

or West Nile fever : the illness caused by West Nile virus

West Nile virus is such a disease. Last week, President Clinton declared an emergency in areas of New Jersey affected by the virus …

—James Ridgeway, The Village Voice

… residents are being urged to stay indoors while trucks spray insecticides through the streets to ward off outbreaks of West Nile fever, a mosquito-spread disease that last summer sickened dozens and killed seven.

—Joannie Schrof Fischer et al., U.S. News & World Report

see west nile encephalitis